Weird captcha code

This tiny captcha code is kind of weird. Only works in Firefox but not IE (6,8)
Hope someone may figure it out what's going on?
Or you may download this code from Author. (demo included)
codefuture.co.uk

/**
* the time() at the end of the address is just to keep the image from being cached
* <img id="captcha_img" src="./captcha/cf.captcha.php?img=<?=time();?>" />
*
*/
if(isset($_GET['img'])){
$capt = new captcha;
// $capt->transparent_bg(false);
// $capt->bg_color('cccccc');
$capt->display();
exit();
}

The demo on Author's website works both on FF and IE.
But when I download the code on local machine. It works only in Firefox.
I thought it's the IE browser setting from the beginning but I already changed all settings and tested under two different OS win7 and xp. It's the same. Only works in Firefox.

Why canít I use certain words like "drop" as part of my Security Question answers?
There are certain words used by hackers to try to gain access to systems and manipulate data; therefore, the following words are restricted: "select," "delete," "update," "insert," "drop" and "null".

I won't claim it's as secure as the normal reCAPTCHA--I haven't researched it thoroughly--but I wanted something I could use (My brain just has trouble with distorted text...), something that felt a bit fresher, and something which I could update if I wanted.